The present invention relates to a sheathed control cable of the type comprising an inner cable, generally made of metal, a cable-guiding duct made of a material having a low coefficient of friction surrounding said inner cable, where appropriate a lubricant such as an oil or a grease between the cable and the cable-guide duct, armoring surrounding the cable-guide duct, and an outer sheath providing mechanical protection, generally made of thermoplastic material.
Such xe2x80x9cBowdenxe2x80x9d cables provide push-pull control and are in widespread use in motor vehicles. As particular, non-limiting applications, mention may be made of cables for manually controlling gear boxes, cables for manual clutch control, accelerator control cables, hand-brake control cables, cables for opening a fuel filler hatch, cables for opening doors and windows, cables for controlling seats, cables for opening the hood, the tailgate or the trunk, and cables for controlling air conditioning.
Apart from applications to motor vehicles, mention can be made of uses for bicycle brakes, bicycle derailleurs, or indeed aviation controls.
Depending on the application, the inner cable may be a stranded cable that is bare or plasticized, a single strand that is bare or plasticized, or it may comprise strands having a flat wire wound thereon.
The armoring may be constituted by an armoring layer for providing compression strength, e.g. in the form of a shell constituted by a flat wound wire, and/or a layer that provides traction strength, e.g. constituted by strands wound helically.
To make the cable-guiding duct, proposals have already been made to use fluorine-containing resins, and in particular polytetrafluoroethylene (PTFE).
The present invention seeks to provide a sheathed cable in which the cable-guiding duct presents improved properties, in particular in terms of endurance and of better quality lubrication of the cable contained inside it, while also providing high efficiency.
In this context, for a given number of cycles, efficiency is determined as being the ratio in percentage terms of the axial load applied to the cable received inside the duct and the force needed to move the cable axially.
In the sheathed control cable of the invention, the cable-guide duct is a duct of microporous PTFE obtained by lubricated extrusion and that has been subjected, on leaving the extruder die, to simultaneous drawing and heating to a temperature above the sintering temperature of PTFE.
The cable-guide duct of the sheathed control cable of the present invention can be obtained in particular by implementing the method described in patent No. EP 0 684 121 in the name of the Applicant company, and it presents specific gravity of less than 2.
Traditionally, a lubricating oil or grease is inserted into the cable-guide duct or is applied to the inner cable before it is put into place inside the duct.
In the invention, it is advantageous to introduce such a lubricating oil or grease into the cable-guide duct during the extrusion process, thus obtaining a prefabricated cable-guide duct making it possible, where appropriate, to omit subsequent lubrication when putting the inner cable into place inside the cable-guide duct.
Naturally, the oil or grease that is selected must be capable of being dissolved in the lubricant used in the lubricated extrusion process and it must be sufficiently stable to withstand the temperature to which the cable-guide duct is subjected during the heating step on leaving the extruder die.
In order to confer a structure on the cable-guide duct that is highly fibrous in the longitudinal direction, drawing is performed at the outlet from the extruder die with a drawing ratio (ratio of outlet speed over inlet speed) lying in the range 1.5 to 3, with this taking place for a duration of 5 seconds (s) to 10 s and at a temperature of 350xc2x0 C.